A typical aircraft powerplant, such as an axial flow gas turbine engine, includes a nacelle which extends about the engine. The nacelle is the radially outermost structure of the powerplant. The nacelle extends forward of the gas turbine engine to define an inlet for working fluid to enter the gas turbine engine. The gas turbine engine includes a flowpath in communication with the fluid inlet of the nacelle. The flowpath extends sequentially through a compressor section, a combustion section and a turbine section. The compression section and turbine section include many components susceptible to impact damage, such as rotating blades which extend across the flowpath and interact with the working fluid.
Safety is a primary concern in the design of powerplants to be used in aircraft applications. One hazard to be avoided is the build-up of ice on the engine and the structure surrounding the engine. The build-up of ice presents many problems. First, ice may add considerable weight to the engine and to the aircraft. Second, the build-up of ice near the inlet of the nacelle may adversely affect the flow of working fluid into and through the engine.
Another concern with powerplants is the useful life of the powerplant and components. The build-up of ice near the inlet of the engine may lead to large pieces of ice breaking loose from the inlet and flowing into the gas turbine engine. Ice flowing into and through the engine may damage components within the engine, such as the blades, and components attached to the nacelle, such as inlet acoustic panels. The damaged components may then require replacement.
Not surprisingly, anti-ice systems for the inlet area of nacelles have been the focus of a significant amount of research and development within the aircraft industry. An example of this is U.S. Pat. No. 4,688,745, entitled "Swirl Anti-Ice System" and issued to Rosenthal. This patent discloses a system for injecting hot gases into an annular cavity located on the leading edge of the nacelle. The cavity extends circumferentially about the inlet. The hot gases are injected into the cavity in a direction tangential to the circumferential direction. The tangential injection produces a circumferentially flowing body of hot gases within the cavity. Exhaust vents or holes located in a bulkhead of the nacelle allow gases within the cavity to escape into the external medium flowing around the powerplant. The venting of the anti-ice fluid prevents over pressurization of the cavity in the event of an anti-ice fluid flow regulator failure. The exhaust holes lie in a plane parallel to the direction of flow to avoid direct impingement of the body of fluid on the exhaust holes.
The anti-ice system disclosed in U.S. Pat. No. 4,688,745 is effective at reducing the built-up of ice on the inlet of the nacelle. There are, however, drawbacks to the use of this anti-ice system. The exhaust fluid removed through the exhaust holes is typically flowed into a chamber within the nacelle. The exhaust fluid exits the nacelle through a vent and flows into the external medium flowing about the nacelle. Flowing the exhaust gases through the nacelle structure may lead to overheating of the nacelle in the vicinity of the chamber.
Further, the fluid exiting the vent tends to be held against the outer barrel skin by the external medium flowing past the outer skin (the free stream). This may lead to overheating of the nacelle surface downstream of the vent. Overheating of the nacelle is especially significant if the nacelle is made from lightweight composite materials. These composite materials are typically bonded together with an adhesive having a lower melting temperature than traditional metallic materials. The rate of temperature decay downstream of the vent is dependent upon the size of the vent and a blowing parameter. Large values for the area of the vent result in a low rate of decay of temperature downstream of the vent. Small values of the blowing parameter, defined as the ratio of exhaust exit velocity to free stream velocity, also result in a low rate of decay of temperature downstream of the vent. The affects of vent size and blowing parameter or the rate of decay of temperature are cumulative.
The vent also introduces aerodynamic penalties in the form of increased drag of the nacelle when the anti-ice system is shut off. The drag caused by the vent is dependent upon the size of the vent and the axial location of the vent. The larger the exit area of the vent or the closer the vent is to the leading edge of the nacelle, the greater the drag attributable to the vent.
A solution to the overheating of the nacelle caused by the exhaust flow through the nacelle is to duct the exhaust flow from the cavity to the vent. Ducting the exhaust fluid through the nacelle structure reduces the likelihood of overheating of the nacelle in the vicinity of the chamber, but also increases the size and weight of the nacelle structure. Attempts to decrease the bulk of the ducts by moving the exhaust vent forward, and thereby shortening the length of the duct, increases the aerodynamic drag of the vent. In addition, the duct does not prevent overheating of the nacelle surface downstream of the vent.
The above art notwithstanding, scientists and engineers under the direction of Applicants' Assignee are working to develop efficient anti-icing systems for the nacelle of a gas turbine powerplant.